”They’re not monsters,” explains the kindly paleontologist Alan Grant (Sam Neill) as he watches a long-necked brachiosaur gulp down a tree limb for breakfast. ”They’re just animals.” Indeed, the dinosaurs who roam, fly, and gnash their oversize teeth through Steven Spielberg’s Jurassic Park (PG-13) are miraculously organic creatures: beady-eyed lizards as realistic as anything you’d see in a zoo. The thrill of the movie, of course, is that they’re monsters, too. The brachiosaurs, with their beautiful, looming necks, seem otherworldly, as magically tall as skyscrapers; the Tyrannosaurus rex is a nightmare of brainless aggression, a prehistoric sci-fi Jaws. Spielberg understands that dinosaurs are God’s monsters, biological marvels that inflamed our imaginations as children — and long after that — because they were like supernatural beasts we could honestly believe in.

In Jurassic Park, adapted from Michael Crichton’s 1990 best-seller, the dinosaurs — some benign, some terrifying, all wondrous — tap into the giddiest science-class daydreams you had as a kid. Created through a blend of computer-generated animation and electronically controlled models, they are so marvelous, and Spielberg choreographs their scenes with such wit, tension, and verve, that it’s easy to overlook the film’s obvious weaknesses: a plot that’s at once busy and thin, characters you like without caring about, a coy layer of blockbuster self-consciousness. As a flight of fantasy, Jurassic Park lacks the emotional unity of Spielberg’s classics (Jaws, Close Encounters, E.T.), yet it has enough of his innocent, playful virtuosity to send you out of the theater grinning with delight.

As rays of blinding white light — Spielbergian light — poke through a dark forest, something large and ominous is crashing through the trees. Is it a dinosaur? No, just a bulldozer. From the opening moments of Jurassic Park, Spielberg toys with our expectations. Before long, we’re off to the desert, where Grant and his paleobotanist girlfriend, Ellie Sattler (Laura Dern), are; digging up fossils. Under the glaring sun, Grant dispels the myth that all dinosaurs were pea-brained. He tells an ominous story about the vicious, cunning velociraptors (”raptors” for short), who would work as a team to outwit their prey. His tale evokes an early scene in Hollywood’s first great giant-monster fable, the 1933 King Kong, and from that moment on the audience is hooked. Spielberg is so good at setting up the wonders to come that he leaves us just about dizzy with anticipation.

Returning to his trailer, Grant stumbles upon a mysterious visitor, John Hammond (Sir Richard Attenborough), a billionaire Scottish developer who has created an extraordinary new amusement park on a tropical island just off Costa Rica. The main feature? Real live dinosaurs, cloned from strands of fossilized DNA. Hammond needs scientific approval of his project to satisfy his investors, so he offers Alan and Ellie a tour of Jurassic Park. Arriving by helicopter at the lush, mountainous island, they are joined by a mercenary lawyer (Martin Ferrero) who represents Hammond’s investors and by Ian Malcolm (Jeff Goldblum), a black-clad mathematician who keeps jabbering on about the perils of tampering with nature. The film wisely treats this old-hat science-fiction theme as a quasi-joke, with Goldblum throwing away his lines in a jittery deadpan.

The characters are sent out on little rail cars to view the creatures of Jurassic Park, which are kept behind electrified fences. Then a thunderstorm strikes, the power goes dead, and the cars stall-right in front of the T. rex paddock. As in Jaws, Spielberg gets you exhilarated at your own anxiety. With its tiny claws and big, bulgy body poised atop birdlike legs, the T. rex is a comically awkward-looking creature, yet it moves with a frightening horizontal thrust. Smashing its huge jaws down on one of the cars (with Hammond’s two grandchildren pinned screaming under a piece of glass), it is so ruthless yet so dumb — a primordial killing machine — that you want to giggle and duck for cover at the same time. Spielberg orchestrates the action with an escalating sense of play. The scene is pure enchantment: scary, majestic, and, at its chomping climax, horrifyingly funny.

In Jurassic Park, each dinosaur encounter is exciting in a different way. At one point, Grant finds himself amid a flock of gallimimus, who are like galloping, earthbound birds. There’s a sickly triceratops, a sprightly dilophosaur that transforms itself like one of Spielberg’s gremlins, a brachiosaur seen in close-up (the only moment that looks a little fake), and, finally, a thrillingly staged battle in which a pair of raptors attack the Hammond grandchildren. These medium-size predators appear unspectacular at first, but their lightning cleverness grows on you. When one of them leaps without warning through a ceiling grate, it seems as lethal as a giant cobra. By the end of the movie, the raptors, like the T. rex, have become true characters, their personalities emerging from their scaly physique.

The best thing about Crichton’s plot is that, to your average scientific know-nothing (like me), the dinosaur-cloning business sounds just plausible enough to tickle your sense of fantasy. The worst thing about it is that the very idea of Jurassic Park, a place where eye-popping wonders are served up as a megabuck attraction, seems an obvious yet pointless metaphor for the commercialization of Steven Spielberg’s empire. Since Hammond’s toys and gizmos feature the same logo that’s being used to sell the movie (and its many tie-in products), there’s no way to separate Spielberg’s ”satire” of marketing from the marketing itself. I wish he’d just dropped the whole Jurassic-Park-as-capitalist-dream business and added more dinosaurs.

Spielberg’s peerless twin gift has always been for making the fantastic seem real (Close Encounters, E.T., the Indiana Jones trilogy) and the real fantastic (Jaws). Recently, though, in movies like the garishly mediocre Hook, he seems to have lost his center, tossing whiz-bang moments around like so much confetti. Jurassic Park has some of that scattershot, impersonal approach — Neill sports an Indy Jones hat but barely registers as a hero — yet the film is held together by the authentic wonder we feel in the presence of its splendiferous creatures. In their ruthlessness and monstrous grandeur, the dinosaurs in Jurassic Park are viewed as our symbolic ancestors: pure embodiments of the primal life force. They’re of this earth but out of this world.